Prior art logic state analyzer displays have merely provided formatted tabular listings or certain vector mappings. None of these displays make it easy to detect a channel or input signal that is apparently inactive, say, because a clip lead from a probe pod to the circuit under test has come loose. Another cause of inactivity might be a genuine circuit failure.
The reason that none of the prior art displays made it easy to spot an inactive signal is that they generally involve some sort of encoding such as BCD, BCO or hexadecimal. A single unchanging bit in the uncoded value merely prevents the coded value from assuming certain values, not from changing at all. Since an incorrect change is still a change an inactive signal does not readily reveal itself through inspection by the user.
Even in a tabulated binary listing where each primary term is available for inspection without having been encoded it would still be necessary to scan each entire column until at least one transition was found in order to verify the presence of activity on each signal. And even then, such a visual check is based solely upon the displayed information, not upon all the information the analyzer has been monitoring, regardless of whether those states were captured or displayed or not. It would therefore be advantageous if the logic state analyzer itself provided an indication of a minimum level of activity for each input signal.
According to a preferred embodiment of the invention described below each input signal is periodically sampled and the sample compared with previous samples to determine which signals have changed in value and which have not. Any failure of a signal to change will cause an annunciator associated with each input signal to display a symbol (or perhaps the absence of a signal) indicative of "no activity". To provide compatibility with refreshed CRT displays where the rate at which a displayed symbol can change is limited by the refresh rate anyway, and to present a more readily perceived change in the values of the annunciators, some convenient number of the periodic samples are "bundled together" into consecutive samples treated as follows. Assume a convenient number is one hundred. Then a signal must change state within each of the one hundred consecutive periodic samples or the entire bundle is "ruined" and the associated annunciator displays "no activity" for a discernible length of time (i.e., at least for as long as it takes to obtain the next "bundle").